Labour secured 262 MPs at the June 8 General Election, up from the 232 elected in 2015 while Ed Miliband was leader.
May will host Arlene Foster in Downing Street to discuss terms of the DUP's backing for her minority government, reports the BBC.
"Experience shows us that unionists have minimal influence on any British government". May had called the vote early in hopes of strengthening her majority going into talks on exiting the European Union, but instead found herself in negotiations with a small party in Northern Ireland in order to stay in power.
Labour's unexpectedly strong second-place showing has thrown national politics into disarray.
Labour leader Jeremy Corbyn countered with a bit of previously unseen swagger, wearing a huge red rose - his party's symbol - in his lapel as he sparred with May and taunted her about the uncertainty surrounding the upcoming vote on her legislative program, known as the Queen's Speech.More news: Family seek 12-year-old girl missing after Grenfell Tower fire
Talks come after Mrs May told Tory MPs: "I'm the person who got us into this mess and I'm the one who will get us out of it".
"It's going to be hard, there's no doubt about that, but perhaps an opportunity to consult more widely with the other parties on how best we can achieve it", he said at a conference in Poland, the Financial Times reported.
It's understood that Mrs Foster won't be returning to Belfast tonight as planned.
"If others decide that they are not coming back into the devolved administration here in Northern Ireland then those issues will have to be dealt with at Westminster", she said.
Foster's rivals in Northern Ireland, such as Sinn Fein's Gerry Adams, have objected.More news: Lawmakers talk more security, packing a gun after shooting
He said he was "concerned" about the deal between the two parties, and saying he was "wary" and "dubious" about it "both for peace process reasons but also for others reasons as well".
The stakes for May are high.
Theresa May insisted the Government was "absolutely steadfast" in its commitment to the Northern Irish peace process as she faced questions on whether a DUP-Tory alliance would put fragile agreements at risk.
"My preoccupation is that time is passing, it is passing quicker than anyone believes because the subjects we have to deal with are extraordinarily complex", he added.
"We stand at a critical time with those Brexit negotiations starting only next week - I think that stability is important".More news: E3 2017: Two New Metroid Amiibo Revealed
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